Four billion. That, according to a new study shared with CNBC by market research firm SimilarWeb, is how many monthly page visits Facebook has shed in a slow-drip but nevertheless huge decline over the last two years.

Facebook’s traffic hasn’t just fallen by about half since 2016, according to the study. Among the consequences of such a precipitous drop is the opening it’s given to YouTube, which the study’s data shows is about to overtake Facebook to become the second biggest site, traffic-wise, in the U.S. Which would give Google ownership of the top two spots, pushing Facebook down to number three.

CNBC describes the drop at Facebook as “severe” and goes on to round out its list this way: “The five websites receiving the most traffic in the U.S. in the last several years have been Google, Facebook, YouTube, Yahoo and Amazon, in that order. However, Facebook has seen a severe decline in monthly page visits, from 8.5 billion to 4.7 billion in the last two years, according to the study. Although Facebook’s app traffic has grown, it is not enough to make up for that loss, the study said.”

Google trouncing Facebook is a bit ironic, considering it was Google’s attempt at a feed-based social network (Google+) that ended up falling flat, when it turned out Google already owned a social winner this whole time in the form of YouTube.

It’s also one more suggestion that maybe, finally we’re hitting “peak social.” (We can only hope, for the good of mankind.)

Snapchat’s parent, of course, reported earnings this week and acknowledged a drop in daily active users for the second quarter compared to the year-ago period. Facebook and Twitter also posted declines during their most recent earnings presentations.

Snap, for its part, blamed shedding users on a much-maligned redesign of the ephemeral messaging app. Facebook and Twitter also blamed the European Union’s new privacy law as part of the reason their numbers are down.

Facebook losing such a large amount of traffic over the last few years is part of a broad realignment in the social media landscape that’s only going to continue, likely with surprising outcomes. It’s also apparently creating buying opportunities.

In a blog post earlier this year, BTIG speculated that Twitter is going to be acquired at some point this year. “Twitter’s 2017 comeback has been fueled by refocusing the company on its core product (iterating features faster than ever before and breaking its own legacy product rules such as #140), creating robust video advertising opportunities for brands and most importantly, making sure users have a better experience every time they visit (by showing them tweets they are interested in and reducing the visibility of trolling), which increases the desire to come back to Twitter more often,” BTIG notes. “With Twitter still in the early stages of its recovery, the time is ripe for an acquisition in 2018 and there is no controlling shareholder that could prevent a transaction.”